The invention relates to a heatable arrangement for producing a gas flow of an inert gas which is enriched with the vapour of at least one low-volatile, pulverulent substance, the arrangement
comprising a vessel having an interior space for holding a powder, PA1 a feed pipe for introducing the gas flow into the vessel and a discharge pipe for removing enriched gas flow from the vessel, PA1 the feed and discharge pipes each having a valve, PA1 the pipes ending in the vessel in such a manner that when the device is operative, the gas flows through the powder, PA1 the vessel (1) and the discharge pipe (19) being provided in a thermostatically controlled bath (2), PA1 the powder (13) including an additional solid inert component, PA1 the interior space (12) having two walls which are arranged transversely to the gas flow (4, 16) and are formed by gas-permeable, porous plates (10, 14), while the other walls of the interior space are impermeable to the gas flow. PA1 the cross-sectional area of the interior space of the vessel is at least a hundred times larger than the cross-sectional areas of the feed and discharge pipes, PA1 one of the two plates, the gas inlet plate, is arranged between the orifice of the feed pipe and the powder, PA1 the other plate, the gas outlet plate, is arranged between the powder and the orifice of the discharge pipe, PA1 at least one of the two plates is movable such, that, during operation, it is pressed onto the powder, PA1 the thickness of the gas inlet plate, the thickness of the gas outlet plate, the pore size of the two plates and the spacing between the two plates or the thickness of the powder bed, respectively, in the interior space are chosen such that, during operation, the pressure drop in the gas inlet plate exceeds the pressure drop in the powder bed, the pressure drop in the powder bed is much higher than the pressure drop in the gas outlet plate and the pressure drop in the gas outlet plate is much less than the pressure in the reactor.
Arrangements of this type, which will also afterwards be denoted as a powder evaporator, a powder saturator or a saturator, have for their object to adjust defined gas flows of low-volatile substances, which, for example, are used as starting compounds for reactive depositions from gas phases for example, CVD (chemical vapor deposition). Since commercially available energy flow control values normally require a differential pressure of approximately 300 hPa, they are only suitable for use with substances having vapour pressures of at least 300 hPa at temperatures below 90.degree. C.
To increase the vapour pressure for low-volatile starting materials for CVD, metal-organic compounds (MOV) of the metal components to be deposited are generally used. Normally, the vapor deposition from the liquid phase of the metal-organic compounds or from a solution of the metal-organic compounds occurs at approximately atmospheric pressure. This results in the first place in low transfer rates and in the second place in a decrease versus time of the flow because of an increasing instability above the melting point and/or in the third place in a co-transfer of the solving agent. For that reason the invention relates inter alia to a powder evaporator for metal organic compounds operating at a low pressure, which solves the problems occurring in fluid evaporators for metal-organic compounds, which are operated at approximately normal pressure.
The problems occurring with low-volatile starting materials will now be described in detail by way of example, with thorium. There are no thorium compounds having vapour pressures of 300 hPa at room temerature. Thus, the thorium halides, for example, which would normally be suitable for CVD-methods do not reach vapour pressures of 10 hPa until at temperatures from 600.degree. C. to 700.degree. C. Certain metal-organic thorium compounds reach approximately 1 hPa at approximately 150.degree. C., have below their melting point a still adequate stability, are anhydrous and can be used in the pulverulent form. Thorium heptafluordimetyl octadionate (Th(fod).sub.4), for example, has a vapour pressure of 0.13 hPa at 140.degree. C, while thorium trifluoracetyl acetonate (Th(tfa).sub.4) has a vapour pressure of 1.3 hPa at that temperature. These compounds cannot be heated to a much higher temperature, as they then already start to decompose.
One of the most important conditions for the use of low-volatile pulverulent starting materials in the reactive deposition of layers from a gas phase at low pressures (LPCVD=low pressure CVD) is that an adequately high and constant energy flow of these materials must be obtained over prolonged coating periods, which are precisely the periods required for a low-volatile material. Almost all the known powder saturators and also fluid evaporators which no longer contain an adequately stable metal oxide compound show a comparatively significant drop in their efficiency and in the flow, although, as far as quantity is concerned, a very adequate quantity of material is available for evaporation and for transfer to the CVD reactor.
An arrangement of the type defined in the opening paragraph is disclosed in the DE-PS 1221520. In the interior of the evaporation vessel disclosed there is a sieve, on which, during operation, the pulverulent, low-volatile material is located in the form of a powder bed. A feed pipe leads from an inert gas storage container to the evaporation vessel, in which the feed pipe ends below the sieve. From the interior space of the evaporation vessel above the sieve a vapor discharge pipe leads to, for example, a reactor, in which vapour plating occurs, in which consequently a CVD-procedure is performed.
From DE-OS 3136895 another evaporator is known in which the feed and discharge pipes are each provided with a valve which is in the form of a non-return valve or a safety valve. From a formula mentioned therein it follows that the energy flow (designated vapour through-flow in said patent) is the higher the lower the overall pressure. In an approximation following therefrpom the overall pressure corresponds to the trigger value; the lowest differential trigger values being however located, for commercially available non-return valves, approximately 30 hPa, so that the overall pressure must not be less than 30 hPa. In addition, the approximation mentioned in the DE-OS 3136895 is not permissible, as the overall pressure corresponds to the sum of the trigger value and the reaction pressure, wherein reaction pressure must be understood to mean the pressure in the CVD reactor. However, also in LPCVD methods the reaction pressure is of the order of magnitude of some hectopascals.